Silence fell across the 17,000-strong crowd. The three swimmers crouched on the start blocks, waiting for the signal. This was the biggest moment of their lives, something they had been training for, dreaming about, for, in one case, almost two months; the first heat of the first round of the women's 50m freestyle. In lane three was Masempe Theko from Lesotho, who only found out she was coming to compete in London in June. In the next lane stood Nafissatou Moussa Adamou, a 14-year-old from Niger, and, with a personal best over a second quicker than her rivals, the clear favourite. Last, and least in stature, was Adzo Kpossi, from Togo. Aged 13 years and six months, she was the youngest athlete in the 2012 Olympics.

Adamou was first into the pool, but the 14-year-old soon slipped behind Kpossi, who swam a strong first 25m. Theko, somehow, took well over a second to react to the gun, almost as though she didn't actually want to get in at all. Which may not have been so far from the truth, given that afterwards she said her main emotion was relief at having got the whole thing over with. With a start like that, she was always going to struggle. Up ahead of her though, the two young teens were having a titanic battle. Kpossi still had the lead until the final 10m, when Adamou's extra year's worth of training just began to show. She sneaked ahead to touch the wall .36sec ahead of Kpossi. The winning time? 37.29sec, some 14 seconds behind the German swimmer Britta Steffan's world record. Adamou didn't stop in the mixed zone to talk to the press, but marched purposefully past to the warm-down pool, mindful of the fact that as at this stage she was leading the entire field of 73 swimmers and that she should prepare herself for the semi-finals. Sadly, her final position was 71st, Kipossi was 72nd, Theko 73rd. But each of the three knocked more than five seconds off their personal bests. Theko, in fact, improved hers by over seven seconds, an improvement to rival that of Ye Shiwen.

Kpossi did stop, and she struck a strikingly professional tone with the press. "I'm sorry," she explained in flawless English, "but I don't speak English." Moments later she was correcting the bad grammar of one middle-aged man's pidgin-French. Was she excited just to be here? "No, no, no" she said, stamping her foot. "I am used to it. I went to the world championships in Shanghai last year, so this wasn't my first time swimming in a big pool."

It was almost a little diva-ish, until you remembered that even a 13-year-old can spot the press's attempts to patronise them. Yes, she said, she trains at the one pool in her part of the country, at the Hotel Mercure in Sarakawa. No, we didn't need to sympathise with her, because it is really quite a nice place to practise. And with that, she broke into giggles and scampered off down the corridor, arm-in-arm with her 19-year-old friend and fellow swimmer, Assita Touré from the Ivory Coast.

By that point, Theko had just about made it out of the pool. She finished in 42.35sec. Never mind how that measures up next to the 50m world record, it's less than 10 seconds quicker than Steffan's time for the 100m. Theko, 25, was one of the sweetest, most charming athletes ever to talk to the press. She gave us a scoop, of sorts, by announcing her retirement from professional swimming. Or rather, competitive swimming. By her own admission, there was nothing that professional about what she was doing.

Theko is an HR consultant. She had done a little swimming in the past, but gave it up five years ago. Like Kipossi, she too had been given an unexpected call up to the world championships in Shanghai last year, where, again, she came last. And that, she thought, was that. She gave up training and went back to work. Then one day in June a letter landed on her doormat. It told her she had been selected for London 2012. "It was a big shock. I was like 'Umm, Okaay'. It took a while to sink in, but eventually I had to admit that 'OK, I had better go and do some training,' because the whole country was going: "Yeah, we've got a swimmer in the Olympics!" At first she was worried her boss would cause "a big hullabaloo" about giving her the time off, but actually, "he was very supportive". Too supportive, almost. Like a lot of other people in Lesotho, he began to get very excited about her chances.

The Lesotho Times ran a story on her under the somewhat optimistic headline 'Lesotho Eyes Olympic Medal in Swimming'. It pointed out that Theko's personal best "is 49 seconds and 75 mini-seconds", and that "should she win her heat she will be up against the world's greatest and fastest Dutch 50-metre freestyler Ranomi Kromowidjojo", whose own PB of 24.10sec was merely twice as quick. Even the partisan local press had to concede that "Theko's weight might also not be on her side as she has gained more kilos since her last official international competition in September last year."

She had, it would be fair to say, a build that gave us all hope that we could still be Olympians. But then she had only been training for a few weeks, in the mornings before she went to work. She got a little more done during a team training camp in Wrexham.

"It has been nerve-racking," Theko said with a smile. "Part of me wishes I could go and do it again and do it properly, but 60% of me is just relieved it is over. Mainly I wish I could have done better. But it is over now, I can't do anything about it." She had loved the "whole spirit and buzz" of the Olympic Park, and had done some star-spotting. "But I am too shy to go and say hello. I saw Ryan Lochte the other day, and Sun Yang, and I wanted to go and ask for their autographs but I was too shy."

Now, Theko says she will go back to Lesotho, the "beautiful mountain kingdom", as she calls it, take up her day job again and "just enjoy that I have been here". Unlike Kipossi and Adamou, Theko thinks her swimming days are over. She did not beat Kromowidjojo, or even Kipossi, but she is an Olympian. And no one can take that away from her.